NBS MONOGRAPH 11

Conductive Flooring for

Hospital Operating Rooms
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UNITED STATES DEPARTMENT OF COMMERCE • Frederick H. Mueller, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Conductive Flooring for Hospital Operating Rooms

Thomas H. Boone, Francis L. Hermach, Edgar H. MacArthur, and Rita C. McAuliff

Reprinted from the Journal of Research of the National

Bureau of Standards—C, Engineering and Instrumentation,

Vol. 63C, No. 2, October-December 1959.

National Bureau of Standards Monograph 11

Issued March 21, 1960

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington 25, D.C. - Price 20 cents
Contents

Page

1. Introduction - 1

2. Scope of investigation 1

3. Description of flooring samples 1

4. Factors influencing the electrical resistance 3

4.1. Age 3

4.2. Humidity 3

4.3. Surface moisture 4

4.4. Service 4

5. Factors influencing the determination of resistance 5

5.1. Applied voltage 5

5.2. Time 5

5.3. Contact resistance 6

5.4. Electrodes 7

5.5. Frequency 8

5.6. Instruments 8

6. Electrostatic tests 9

7. Nonelectrical properties 10

7.1. Indentation 10

7.2. Scratch resistance 11

7.3. Slipperiness 11

7.4. Scrubbing 12

7.5. Water absorption 13

7.6. Stain resistance 13

8. Summary and conclusions 13

9. References 15

10. Appendix A. Excerpts from the 1958 edition of NFPA publication No. 56,

Recommended safe practice for hospital operating rooms 15

11. Appendix B. Four-terminal method of measuring contact and internal resis- tivities 16

12. Appendix C. Calculation of resistance from contact and internal resistivities 16

31
Conductive Flooring for Hospital Operating Rooms

Thomas H. Boone, Francis L. Hermach, Edgar H. MacArthur, and Rita C. McAuliff

(July 9, 1959)

Characteristics and performance of available types of conductive flooring materials were investigated in the laboratory. The study showed that the electrodes and instruments

used to measure the floor greatly affected the measured resistance, but that the method

specified by the National Fire Protection Association for measuring the electrical resistance

reasonably simulated the conditions under which a floor functions in reducing electrostatic

hazards. The physical, chemical, and serviceability characteristics of conductive floorings

investigated showed results comparable with those of nonconductive flooring of the same

type. Consequently, with some limitations, an architect may base his choice of a conductive

flooring material on his knowledge of the behavior of similar nonconductive materials.

1. Introduction

Sparks which can result from, the accumulation of

jstatic electricity constitute a very real hazard in

jlocations where explosive vapors are present [l]. 1 The most effective of the several possible means

jof mitigating this hazard consists in keeping the

i electrical resistance between all objects in the hazardjous locations so low that dangerous voltages are

[never attained [2]. General quantitative considerJations indicate that a resistance of 30 to 50 million

johms between objects is low enough for this purpose,

(for any rate of separation of charges which can reasonably be expected to be attained by accidental electrostatic processes [1,3]. Most objects normally

I rest or move upon the floor and therefore can be

electrically connected by way of the floor. Flooring

I of sufficiently low electrical resistance (conductive

flooring) is thus of paramount importance in the

elimination of electrostatic hazards. At the same

time, however, the electrical resistance must be high

enough to minimize the possibility of electric shock

from faulty electrical wiring or equipment.

The specification and the measurement of the

[ resistance of conductive flooring are greatly complicated by a number of factors, such as the kinds of

electrodes and the characteristics of the instrument

used. A method recommended by the National

Fire Protection Association (NFPA) has had general,

but not universal, acceptance for routine measurements of installed floors [4,5]. In this method the

resistance is measured with a 500-v instrument

connected to two similar electrodes placed on the

floor.2 With these electrodes, which simulate footwear and conductive rubber objects, the specified

lower and upper limits of resistance are 25,000 and

1,000,000 ohms (0.025 to 1 meg).

At present there are quite a few specially compounded proprietary flooring materials which meet

1 Figures in brackets indicate the literature references on page 15.

2 These requirements and recommended methods of testing floors are repro- duced in full in appendix A at the end of this Monograph.

these upper and lower limits. However, as is

shown in this report, not all of the conductive floors

remain within these limits of resistance under all conditions of use.

2. Scope of Investigation

A conductive floor must provide a path of prescribed conductance for a reasonable life and should,

in addition, possess the physical properties required

of an ordinary nonconductive floor. This investigation was planned: (1) To measure by established

methods the electrical resistance of each sample of

flooring; (2) to determine the effect of all pertinent

factors that might affect its resistance, such as wear,

aging, moisture, and maintenance; (3) to evaluate

the factors that influence the measurement of electrical resistance of conductive flooring by determining the effect of variations in test conditions upon

the measured resistance; (4) to check these results

by direct tests in which the flooring serves to reunite

electrostatic charges; (5) to compare the significant

physical properties of each sample with those of nonconductive floors of the same type. In addition to

the tests conducted in the laboratory on samples of

flooring, field tests of five different types of conductive

floors were made in the Washington area. Although this investigation was carried out primarily to evaluate conductive floors for hospital

operating rooms, many of the results obtained should

apply equally well to floors for eliminating electrostatic hazards in other locations such as munitions

plants and munitions storage depots.

The authors realize that an additional important

property for floors, particularly in hospitals, is that

they be made and kept reasonably germ-free. Tests

to determine differences in this respect were beyond

the scope of this study.

3. Description of Flooring Samples

The samples for this study were obtained from

domestic suppliers of commercially available con1
I

ductive flooring. By request, each manufacturer

supplied five similar 18-in. by 18-in. floors on plywood panels. The thickness of the floor, bonding

technique, and any reinforcement and conductive

intercoupling were of the manufacturer's own practice. Thus, each sample received was assumed to

be of the type and construction considered by the

manufacturer or his trade association as most suitable for conductive flooring.

Preliminary tests of several types of floors with

electrodes spaced 1, 2, and 3 ft apart, and an analysis

of the effect of sample size and shape (see section

5.3), showed that the resistance is not greatly affected

by electrode spacing. Therefore, convenient 18-in.

by 18-in. samples, which permitted a 1-ft spacing

between electrodes, could be used. The preliminary

study showed that the results should be very close to

those which would have been obtained had larger

samples or even an entire floor been used, with the

3-ft electrode spacing specified in NFPA 56.

The electrical conductivity of each sample, except

the oxychlorides and one make of ceramic tile, depends either completely or partially on the presence

of acetylene black (carbon). This is a special form

of carbon black produced by the thermal decomposition of acetylene gas under carefully controlled conditions. In the case of the ceramic, linoleum, rubber,

and vinyl samples, the carbon black is finely dispersed

in the material during manufacture, while in the case

of latex, concrete terrazzo, and the setting bed ce- ment for ceramic tile, the carbon black is uniformly

dispersed in the dry powder mixes, placed in containers, and shipped for on-the-job composition.

Oxychloride floors are made by combining an aque- ous solution of magnesium chloride with powdered

magnesium oxide. Various fibrous and mineral fill- ers are mixed with the resulting paste which sets to a hard mass. Marble chips may be added to the mix

and the surface ground to produce a terrazzo floor.

Brief descriptions of the composition and installation techniques of each of the conductive floors tested

follow

Ceramic, Sample No. 1: This flooring consisted of

an attractive "block random, sprinkle pattern." The

design had been made with black conductive tiles,

1%6 in- square, % in. square, and 1%6 in. by % in. oblong.

A nonconductive green oblong tile, % in. by % in.,

was also inserted. The K-in. -thick tiles were laid in a conductive mortar underbed containing 3 percent

acetylene black (carbon). The joints were grouted

with nonconductive cement mortar and were approximately Yu in. in width.

Ceramic, Sample No. 2: This flooring of brown

conductive tile, l%e in. square by }i in. thick, had

been laid the same as sample No. 1. Samples were

also submitted with the tile set in a conductive

adhesive using nonconductive cement mortar joints.

Coating, Sample No. 3: This flooring had been

made from a mixture of plastics, solvents, and con- ductive ingredients, the mixture having been applied

by spray, brush, or trowel to a thickness of %6 in. It was supplied in black.

Concrete, Sample No. 4: The sample was submitted

as representative of conductive concrete terrazzo.

{ Detailed specifications have been published by the.

National Terrazzo and Mosiac Association for laying

carbon black conductive terrazzo floorings for operating suites of hospitals. The sample had been con-;' structed according to these specifications, which included a concrete underbed containing 3 percent car-i

bon black and a terrazzo top surface with 2 percent

carbon black. The amount of carbon black was based

on the weight of dry cement. The matrix had a dark

gray color with stone chips of black and green. These

floorings were treated with a recommended pene-!

trating type sealing compound.

Latex, Sample No. 5: This sample was a mixture of. a neoprene latex and cement binder with pink, green,

and white chips. The material had been troweled.

% in. thick and was intended for use over existing! or new structurally sound underfloors. As carbon j black was used as the conductive medium, only black

j was available as a matrix color. As in other terrazzo

design floorings, various color combinations can be)

achieved by use of different colored chips. Approximately 60 percent of the surface area was covered

by nonconductive chips. The manufacturer had'

applied four coats of his recommended sealer.

Linoleum, Sample No. 6: This flooring was black'

and was available in 6-ft-wide strips, }i in. thick.*

The linoleum had a burlap backing and could be

placed over a suitable underfloor by conventional \ methods of installing linoleum. The manufacturer?

prescribed brass seam connectors with projecting)

points for the purpose of electrical intercoupling \ between sheets. The manufacturer also stated that)

wax or protective coatings in any form should not

be used, and recommended a dry machine brushing

to produce a polished appearance.

Oxychloride, Sample No. 7: The conductive ter- razzo flooring had been laid % in. thick over suitable

underfloor. A liquid synthetic resin bonding agent, over which coarse mineral grains were spread, provided the anchoring between the top terrazzo surface

and the underfloor. The matrix was green and}

approximately 50 percent of the surface area was I covered with black and white nonconductive chips.)

Oxychloride, Sample No. 8: This sample had beeni

installed in the same manner as sample 7 except)

that the top %-in. coating was a plain, dark red,)

trowel finish.

Oxychloride, Sample No. 9: The conductive ter- razzo flooring had been laid % in. thick over asphalt

felt and wire mesh with a suitable underfloor. The

matrix was white and approximately 53 percent of

the surface area was covered with black and white;

nonconductive chips. The surface was coated with|

a sealer.

Oxychloride, Sample No. 10: The conductive ter- razzo flooring had been laid }{ in. thick over 2-in.-j

square wire mesh and a suitable underfloor. The

matrices of the samples received were red, green, and 1

gray, all with approximately 30 percent of the surface !

area covered with black and white nonconductive i chips.
Oxy'chloride, Sample No. 11: The conductive ter- razzo flooring had been laid % in. thick over suitable

underfloor. The matrix of this sample was green

with approximately 65 percent of the surface area covered with black, green, and white nonconductive

chips.

Oxychloride, Sample No. 12: The plain, trowelfinished, conductive flooring had been applied % in.

thick over a suitable bonding agent and a suitable

underfloor. The sample received was red, but other

colors are also available. This so-called cupric

oxychloride material differed from the other oxychlorides in that it contained finely divided copper

powder (5-10% by weight of dry mix), which was

claimed to impart a number of desirable characteristics. The manufacturer stated that the flooring

should not be waxed, and recommended the use of a special sealer.

Rubber, Sample No. 13: This flooring consisted of

rubber, homogeneously compounded with acetylene

carbon black. The material was black, % in. thick,

with a cotton fabric backing. Adhesive was used

to fasten the sheets to a suitable underfloor and

intercouplings similar to those used with linoleum

(sample no. 6) can be used to connect the sheets of

rubber electrically.

Vinyl, Sample No. 14: This flooring consisted of

9-in. by 9-in. polyvinyl chloride-based tiles, % in.

thick, with a black conductive field and a white and

green marbleized design. Installation was effected

with a special underlayment felt which carried its

own pressure-sensitive adhesive on both sides, thus

serving to bond the felt to the underfloor and the tile

to the felt. Copper foil, % in. wide, was placed on

the felt to provide an electrical intercoupling between

tiles. Vinyl, Sample No. 15: This flooring consisted of

9-in. by 9-in. polyvinyl chloride-based tiles, in.

thick, with a molded terrazzo design of either a white or gray field with a black "chiplike" effect.

Installation was by troweling adhesive onto a suitable underfloor and placing 1-in. copper foil on the

adhesive to provide an electrical intercoupling between tiles.

4. Factors Influencing the Electrical

Resistance

The changes of electrical resistance of the sample

floorings when subjected to a practical range of en- vironmental conditions were determined. Based on

previous knowledge of the behavior of some similar

types of floorings, the following were selected as the

most important factors : aging, moisture, and maintenance procedures.

All of the resistance measurements were made in

accordance with section 6-2 of NFPA method No. 56

(see app. A) except for reduced spacing of electrodes. This established test method provided a good basis for comparison of these floors, and, as shown in subsequent sections, proved to be suitable

and realistic.

4.1. Age

In order to determine what changes may occur in

the electrical resistance of conductive flooring as it ages, resistance measurements were taken over a

period of 30 months for all samples except sample 4,

which was received much later than the others.

During this time the samples were exposed to 50 ± 2

percent relative humidity (rh) and a temperature of

22° ±1° C. At intervals four measurements were

made at four different positions on each panel with

the electrodes 10 in. apart. A template was used to assure that the electrodes were always placed on the

same areas of each sample. The graphs (fig. 1)

illustrate the electrical resistance of each sample as a function of time. As indicated in figure 1, aging

did not significantly affect the resistance of the

floors except for some oxychloride samples.

to5

xo

o

s

to

LU

EC

1 1 CERAMIC t1 1

1 CONCRETE

.-CONCRETE / TERRAZZO

CERAMIC 1

i

1 1 1 1

n 1 i r

LATEX 8 COATING

I I L

6 12 18 24 30 6 12 18 2 4 30 TIME, MONTHS

SOFT COMPOSITION

24 30 6 TIME, MONTHS

Figure 1. Effect of aging on electrical resistance as measured

by the NFPA method.

Ambient temperature 22°±1°C, relative humidity 50 percent.

4.2. Humidity

Although the relative humidity in some hospital

operating suites is controlled automatically by air

conditioning equipment and thus can be maintained

at any desired level, such equipment is still generally

lacking, and most conductive floors are therefore
exposed to wide variations in relative humidity.

Tests were conducted to determine the effect of ex- tremes of relative humidity on the electrical re- sistance of the flooring samples. The samples were

exposed to an atmosphere of 10 ±1 percent rh at

38° ± 1°C for 3 weeks and to an atmosphere of 80 ±1

percent rh at 27°±1°C for 1 week. In the interval

between the two tests they were stored at 22° ±1°C

and 50-percent rh for 2 weeks. The electrical re- sistance was measured at the beginning and end of

each of the two tests and for several days after the

conclusion of each test. The results of these tests are shown by the solid lines in figure 2. As is evident

from these graphs, only the oxychlorides were materially affected. It is evident that the electrical

resistance of oxychloride flooring is dependent on its

moisture content. As shown by the graphs, ex- posure of the oxychloride samples to 80-percent rh

caused then resistance to fall below the 25,000-ohm

minimum and exposure to 10-percent rh caused their

resistance to go above the 1-meg maximum.

The effect of humidity on electrical resistance was

pointed up secondarily in an investigation whose

RELATIVE HUMIDITY

1.0

0.10 -

0.025

0.01

1.0

052o

<(-

if) UJ

or

0.025

0.01

0.001

'NO. I

1 CERAMIC

' NO. 2

1 CERAMIC

' NO. 3' COATING

' N0.4 1 CONCRETE TERRAZZO

1: 1: 1: -: 1: h

I! •

li I:

'no. 5' LATEX TERRAZZO

' NO. 6' LINOLEUM

II 'no. 7'

I OXYCHLORIDE

1 'N0.8

1 OXYCHLORIDE

1: 1: | = 1 : 1 : 1

:

1

1 1: 1 1

0.025

0.01 -

1

1 NO. 9

' OXYCHLORI0E

1 'no.io' OXYCHLORIDE

'no. 11

'

| OXYCHLORIDE

'no. 12' OXYCHLORIDE

Ih

—

: 1

III.'

1 1 1

1

1.0

0.10

0.025

'no. 13' RUBBER

'no. 14' VINYL

'N0.I5

1

VINYL

1: 1: Is"

IS 1: 15. I: 1: IS.

1 '

WITHOUT

WASHING

WITH DAILY WASHING

Figure 2. Effect of ambient relative humidity and daily

washing on electrical resistance as measured by the NFPA

method.

primary purpose was the study of the effect of alkali'; or lime content on the electrical resistance of various

cement mixes. A few scattered measurements, made,1 a number of years ago, had indicated that concretes;

made with high-alkali cements apparently had lower|

electrical resistance than those with low-alkali or high-lime content. Because conduction in concrete:

is electrolytic, this seemed to warrant further investigation, since ordinary concrete is not far above the

present specification limit at moderate humidities.'

The results (table 1) verified that high alkali concrete does have somewhat lower resistance than the

other types, but the difference is not marked andj,

may be due to unknown factors. Table 1 also; illustrates the effect of humidity on the electrical

resistance of ordinary concrete (without carbon)

black)

.

Table 1. Electrical resistance of concrete specimens

NBS No. Alkali CaO

Resistance

At 15% rh At 50% rh

1 HA %

0.89

.91

.91 .13

.08

.30

% meg

7050

70 400

150

400

440

80

90

meg

8

2 HA.... 11960

30

5080

20

15

3 HA __

1 LA...

2 LA.._

3 LA_

1 HC 65.0

63. 5 63.3

2 HC

3 HC

cement. The samples were 1- by 1- by 11-in. concrete bars made with three t different types of cement, each from a different supplier. All were of 1:1:2 mix L (cement, fine, and coarse aggregate, respectively) with a cement-water ratio of t about 36%. The samples were damp-cured and were then placed in a controlled |' humidity cabinet, first at 15% rh and then at 50% rh, at room temperature.

f The resistance of each was measured periodically with a 500-v d-c instrument

f between two resilient electrodes, one at each end of the same surface of the bar.

[

4.3. Surface Moisture

Since the floors of a hospital operating room are naturally kept as clean as possible, tests were con- ducted to determine the effect of routine maintenance

(i.e., water mopping) on the electrical resistance of

the samples. The samples were mopped at 9:00 a.m. daily for 1 week with a rubber sponge saturated

with water, and any excess water was allowed to

remain. The electrical resistance was measured

daily at 1 :00 p.m. Three series of such tests were

run, one at 10 ± 1-percent rh (38° C), one at

50 ± 2-percent rh (22° C), and the third at 80

± 1-percent rh (27° C). The effect of these tests

on the electrical resistance of the specimens is

indicated by the dashed lines in figure 2. Again,

the oxychlorides were the only samples that were

materially affected, their resistance falling below

the minimum permissible limit in all cases except

in the test at 10-percent rh.

4.4. Service

In order to determine the degree of correlation

between laboratory tests and tests of some conductive flooring in actual service, field tests were made

4
jof floors in several hospitals in the Washington area. |The results are shown in table 2. They indicate

Reasonably good correlation between the measured

, resistances of the samples and the installed floors

junder roughly comparable conditions.

Table 2. Comparison of field and laboratory tests

Type of floor

fllTTl ilfl T* OllLlllCAL IU sample No.

Range of electrical resistance

Field Laboratory (rh 10%

to 80%)

1 meg meg

beramio 1 0.085 to 0.20 0.050 to 0.13 Concrete terrazzo 4 .12 to 1.5 .050 to .33 (Concrete terrazzo 4 °.80 to 9.0 .050 to .33 [Linoleum 7 .025 to .045 .037 to .055 'Oxychloride 9

b i.O to 10 or more .012 to 10 or more pnyl._- 15 .048 to .068 .17 to .26

» The installation method now specified by the National Terrazzo and Mosaic

| Association would probably eliminate this type of failure. | ' This floor had been out of service for about three months and had not been washed regularly.

5. Factors Influencing the Determination of

Resistance

The established method given in NFPA No. 56

(see appendix A) for measuring the electrical resist- ance of installed floors represents a series of compromises among several conflicting requirements.

These have been necessary because, unlike metallic

conductors, the resistance of the usual conductive

| flooring material depends upon how it is measured.

I It is greatly dependent upon such factors as voltage

gradient, type and shape of electrode, and time and

frequency of applied voltage. The large number of

floors submitted in this program made it possible to

evaluate these factors on a much better basis than

I heretofore.

The fundamental principle used in these studies

was that a material whose properties depend upon

the conditions of measurement should be measured

by methods which simulate as closely as practicable

the conditions under which the material is expected

to function. The tests were intended to evaluate

this similarity for each of the effects which prior

experience had indicated were significant.

5.1. Applied Voltage

One of the most important of these effects is that

of applied voltage. For most conductive flooring

materials conduction is either by the migration of

ions (electrolytic) or by complex chains of particles

of carbon in a nonconducting matrix. In either

mechanism it would be expected that the magnitude

! of the applied voltage would affect the measured

! resistance. To determine this a Wheatstone bridge 3 was used to apply a d-c voltage to each sample and

3 A commercial megohm bridge with an electronic detector was modified for these tests to measure resistance down to 0.001 meg (1,000 ohms) and to provide an

adjustable bridge voltage. It had an effective internal resistance of 50,000 ohms

to limit the current for the protection of the operator and to avoid overheating

low-resistance samples. This affects the voltage applied to the sample, as shown

in section 5.5. In these tests the voltage actually applied to the sample was

measured.

to measure the corresponding resistance. The voltage was increased in steps of 50 v to the maximum

output voltage obtainable (limit 500 v.).

The results for typical flooring materials are given

in figure 3. They show that for most materials the

resistance is approximately an exponential function

of the applied voltage, described by an equation of

the form R=kVn

. The exponent, n, ranged from

0 to —2.

Experiments have shown that a minimum voltage

of about 400 v is required for an electrostatic spark

in air at atmospheric pressure [2]. Thus the principle just mentioned indicates that a voltage near this

should be used in evaluating floors for locations

such as hospital operating rooms, in which ignition

of flammable gases by sparks is the principal hazard.

Such floors should not be measured by ordinary

ohmmeters which apply only a few volts to the

sample. Fortunately 500-v insulation-measuring instruments (direct or ratio ohmmeters) are readily

available.

20 30 50 \IOO 20O 300 500

APPLIED VOLTAGE, VOLTS

Figure 3. Effect of applied voltage on resistance as measured

by the NFPA method.

5.2. Time

The conventional method of measuring the electrical resistance of an installed floor, for example the

method specified in NFPA No. 56, is, in effect, a "steady state" method in which the resistance is

measured several seconds after the direct voltage is applied. In practice, static electricity is "generated" (charges are separated) by motions, such as

5
removing a sheet from a table or getting up from a

chair, the durations of which are comparatively

short, generally in the range from 0.01 to 1 sec [1]. Thus it is the short-time resistance or "surge resistance" of a floor which determines the rate at which

the separated charges reunite. For some types of

polarization of flooring materials this initial resistance

may be less than the steady-state value.

The initial and steady-state resistances of a number of types of flooring materials were evaluated

with a cathode ray oscillograph in the circuit shown

in figure 4. The oscillograph was connected across a resistor of low inductance to record the current

through the sample when the switch, S, was closed.

The oscillograph was triggered by the same switch

and operated in the single sweep mode with a sweep

time of about 30 msec. After 3 sec the current was

read from the milliammeter and the sweep was

again repeated Avithout interrupting the current.

The ratio of the initial resistance to the steady-state

resistance was computed from the initial deflection

of the oscillograph (generally observed about 1 msec

after the switch was closed), and the deflection at

3 sec.

Figure 4. Laboratory circuit for measuring initial and steadystate resistance.

B, NFPA standard electrode; R, a-c resistor; CRO, cathode-ray oscilloscope and camera, connected to R with coaxial cable, shield at ground potential; T,

leads to sweep trigger circuit of CRO; I, milliammeter.

Tests were made at battery voltages of 150 and

400 v, at an ambient temperature of about 25° C,

and a relative humidity of 50 to 60 percent. The

results are shown in table 3. In most cases the

observed difference between the initial and steadystate deflections was small. On this basis it is evident that the 3-sec values of resistance as presently

specified for routine tests are not significantly in error and that the fractional changes in resistance

for shorter times are not unduly large for the flooring

samples tested.

Table 3. Ratio of initial resistance, Ri, to steady-state

resistance, Rf, of flooring samples (standard electrodes, relative humidity 50-60%)

Sample Type Rf RijRt

meg

1 Ceramic- --- - 0. 10 0.8 2 Ceramic -- - - --- - .08 .7 3 Coating

_

_ .04 .9 6 Latex. ... --- .08 .8

6 Linoleum .11 1.0

11 Oxychloride .04 0.8

13 Rubber. . __ .20 .9

14 Vinyl. .23 .8

15 Vinyl.... .41 .8

5.3. Contact Resistance

Experiments have shown that most flooring,

materials exhibit surprisingly large "contact rei

sistances" at the interfaces between the material

and the electrodes which are used to measure the!, resistance. Because of this the two-terminal method)

of testing installed flooring (using two electrodes!

does not measure the true internal or volume re'-j sistance of the flooring material. When necessary;!

the contact and volume resistivities are best de-j

termined by making four-terminal measurements irl - which two "current" electrodes are used to carry

current to the sample under test, with two separate*

"potential" electrodes between them, connected to* a voltmeter whose resistance is much greater thanf

the resistance of the sample. If the electrodes arei

(

0( placed on a rectangular sample as shown in figure1

5, the potential electrodes do not disturb the poteD-|<

tial gradient in the sample. Then, if the resistance^ f

between the current electrodes, the applied voltage,

and the voltage between the potential electrodes are measured simultaneously, the average contact

resistivity and the internal resistivity can be com

puted, as shown in appendix B.

I '0

J L

it

i

\

I

Figure 5. Four-terminal method of measuring contact and

volume resistance.

Several typical samples 4 were measured by this

method with the 500-v megohm bridge. The direct'

voltage from the bridge was applied to two barshaped current electrodes, 1% in. wide and 18 in.

long (faced with rubber and covered with foil like

NFPA electrodes), with the potential electrodes!

between them. An electronic voltmeter with an

input resistance of 10 12 ohms was used to measure!:

E. The results are shown in table 4. From this

table it is apparent that for each of these materials

the contact resistivity with this type of electrode

is much greater than the internal resistivity.

It has been argued that the test of an installed:

floor should be a measurement of the true internal!

* Samples which had an embedded wire mesh could not of course be evaluated

j by this method.
Table 4. Contact and internal resistivities

Sample

No. Typo Contact re- sistivity Internal re- sistivity

Computed resistance between NFPA elec- trodes

1-ft spacing 3-ft spacing

1 Ceramic —

Ohm-cm2 3XW

3X105 2X106

2X10« 2X16«

Ohm-cm

7X10' 2X103 9X103 2XW

9X101

Meg

0.2

.03

.1

. 1 .4

Meg

0.2

.04

. 1 .2

.4

3 Coating

5 Latex. . 6 Linoleum.

13 Rubber... .

Relative humidity. 50±5%

resistivity of the material and should be made by

reducing as far as possible the contact resistivity, for

instance by using conductive jelly under the electrodes. However, objects in the operating room

must contact the floor, and the principle previously

outlined requires that the test electrodes should

properly simulate such contacts. In addition, fourterminal measurements show that it is not possible

to eliminate contact resistance completely even

with wetted electrodes.

The separation of contact and internal resistivity

makes it possible to calculate the resistance to be

expected for many different configurations of electrodes. In particular, the resistance between two

NFPA electrodes on a large floor can easily be determined, as shown in appendix C. The values calculated in this way for several of these flooring samples are given in the last two columns of table 4. These show that the measured resistance is not appreciably affected by the electrode spacing because

of the high contact resistivity which is independent

of spacing and because the internal resistance of the

material between the electrodes is a logarithmic

rather than a linear function of the ratio of the electrode diameter to the electrode spacing. Increasing

the spacing from 1 to 3 ft increases the internal re- sistance by less than 50 percent (and does not affect

the contact resistance). In addition, the limited

area of the 18- by 18-in. samples, which restricts the

lines of current flow, increases the internal resistance

and somewhat offsets the effect of the reduced electrode spacing.

The computed results agree reasonably well with

the values measured for the same samples with

NFPA electrodes at approximately the same voltage

and ambient relative humidity. The results of these

measurements are given in the next section.

5.4. Electrodes

Because of the high contact resistivity it would be

expected that the kind and size of electrodes would

have marked effects on the measured resistance of

conductive floors. From the principle already given,

the electrodes should simulate the contacting objects.

In an operating room, such objects are of two general

types: (a) Soft, resilient materials, such as conductive-rubber shoe soles and heels and conductiverubber casters and leg tips on furniture; and (b) hard

objects such as metal leg tips and gliders. The force

538361—60 2

on these objects can be expected to vary over a fairly

wide range, from perhaps a few pounds (light pail)

to several hundred pounds (operating table). Thus

no one electrode can be expected to simulate all of

these objects.

To evaluate these effects, tests were made of typical materials with several different electrodes and

with added weights on each electrode. All measurements were made with the 500-v bridge and at an

ambient relative humidity of 30 to 50 percent. Five

readings, with the electrodes at different locations,

were made on each sample. The average results are given in table 5, along with a key to the types of

electrodes used.

Table 5. Results of tests with different electrodes

Sample

No. Type

Ceramic

Ceramic

Concrete a .

.

Latex Linoleum .

.

Oxychloride

Oxychloride

Rubber

Vinyl

Resistance in meg (see key below)

0. 055

. 11 .026

.051 .051

.022

.023 .24

.16

SW

0.037

.060

.040 .049

.019

.019 .23

. 11

H

0.29

1.4 0.28

.16

.080

.080

. 11 .36

.37

HW

0. 24 .80

.060

.070

.33 .22

0. 33

.65 .30

.22

.17

2.0

0. 25 .53

b 1. 0

CS B

0. 14 .34

.38

. 11 .10

1.0

0. 65

.41

1.7

6.5

0. 85

.85

1.0

Key

Symbol Type of electrode

S Two 5-lb, 2.5-in. diam electrodes, with flat but resilient contact

face on each, conforming to NFPA No. 56. Same as S with 50-lb additional weight on each electrode. Two 5-lb electrodes with flat steel contact faces, 1-in. diam, having rounded edges. Same as H with 50-lb additional weight on each electrode. Two conductive casters, 3-in. diam, H-in. tread. Weight of each about 1 lb with 5-lb additional weight on each caster. One type "C" and one type "S" electrode. Two 5-lb electrodes, each having three-legged contacts of J-s-in. by

}&-m. brass rods, having flat surfaces and sharp edges.

SW

H

HW

C

CS

B -

» Sample tested shortly after receipt, before aging.

b Moderate variations. Some additional readings up to 100 meg.

The results show that for all materials the re- sistance measured with the resilient NFPA electrodes

is not appreciably affected by large changes in force, so that these electrodes, which weigh only 5 lb and

are reasonably portable, can simulate the much

heavier objects likely to be encountered in operating rooms. For resilient floors (such as vinyl tile, rubber,

linoleum, and the coating with the wood backing) the

results with the hard electrodes, which simulate

metal leg tips and gliders, are not appreciably higher

than with the NFPA electrodes. However, with

hard-surfaced materials (oxychloride, concrete ter- razzo, and ceramic tile) the actual contact area was

much less than with the resilient electrodes, and the

measured resistances were greater by factors as large

as 10 or more (and in one case by a factor of 100).

In general, the scattering in the 5 readings of re- sistance with each electrode (only the average values

are shown in the table) was very much greater with

the hard than with the resilient electrodes. The

conductive casters, which have a small contact area,

showed marked scattering in readings of resistance
on some samples. The differences between results

with two standard electrodes and those with two

casters were less marked for resilient flooring (except

for vinyl tile) than for hard-surfaced materials.

(Field tests of a conductive vinyl tile floor of the

same manufacture gave nearly equal results with

casters and standard electrodes.) The special 3-

legged electrodes have been suggested for simulating

high-pressure contacts. The resistances were in

general comparable with or somewhat higher than

those with the 1-in. hard electrodes.

Thus the resilient electrodes simulate well one

type of floor contact and give much more uniform

and reproducible results than hard-surfaced electrodes. However, when such resilient electrodes are specified, the upper limit of resistance for an

installed floor must contain a large factor of safety

to allow for hard-surfaced objects which are also

widely used. This factor depends upon the resiliency

of the floor, so that apparently the specified upper

limit with a resilient floor such as conductive rubber

or linoleum could safely be 10 times as high, when

measured with these electrodes, than the limit for a hard floor such as oxychloride or concrete terrazzo.

5.5. Frequency

The lower limit of resistance specified in NFPA

No. 56 for conductive floors is intended to provide

some (but not complete) protection from electric

shock. Alternating current (120 v, 60 cps) is almost

universally used in this country for electrical utilization circuits. Because of this it would appear that

the resistance of the floor should be measured at

this voltage and frequency to determine compliance

with the lower limit. For convenience, however,

the same d-c instrument is normally used for both

upper and lower limit tests. The instrument commonly specified has an open circuit voltage of 500 v. However, the actual voltage that it impresses on the

floor under test depends upon the resistance of the

floor and the internal resistance of the instrument,

as shown in figure 6. From this figure it is apparent

that a 500-v instrument having an internal resistance

of almost 0.1 meg will impress about 100 v on a sample near the NFPA lower limit of 25,000 ohms

(0.025 meg).

Measurements of several flooring samples were

made at 100 v, with alternating and direct current,

and with a typical 500-v ohmmeter which has an

internal resistance of 0.1 meg. All measurements

were made with the standard resilient electrodes at

the same location on each sample, and at an ambient

relative humidity of about 40 percent. The results

given in table 6 show no really significant differ- ences between the a-c and d-c measurements of a given sample at 100 v. They also show that for

samples near the lower limit of 0.025 meg the ohmmeter value agreed reasonably well with others.

For samples of higher resistances the ohmmeter

impressed more than 100 v on the sample and the

measured resistance was, in general, less, as would

be expected from the observation that resistance is an inverse function of the voltage, as shown in secTable 6. Ac-dc comparison tests

Sample

No. Type

Ceramic

Ceramic

Concrete Latex Linoleum, „

Oxychloride

Oxychloride Oxychloride

Rubber

Vinyl

Resistance in meg

At 100 V At 100 v

60 cps, ac dc

0. 20 0. 23

.31 .34

.009 .024

.21 .25 .055 .057

.042 .038

.012 .007

.063 .040

.21 .23 .60 .78

With I ohmmeter

,

0. 075

1

.10,

.01?

.07? .050

I

. 03C, .oiq

.04C .21 .20,

tion 5.1. The 500-v instrument specified in NFPA

No. 56 (internal resistance 50,000 to 200,000 ohms)!

thus can be expected to give results which are reasonably close to those obtained with the some- what more complex instruments required for a-c; measurements, and make it unnecessary to specify

two different instruments for measuring installed

floors. However, as figure 6 shows, the range ol

internal resistance specified in NFPA No. 56 could

be greatly narrowed. A short circuit current of

5 ±0.5 ma corresponding to a nominal interna'

resistance of 100,000 ohms is suggested.

10,000 100,000 I MEG. 10 MEG.

( MEASURED RESISTANCE , OHMS

Figure 6. Output voltage of ohmmeters as a function of measured]

resistance.

I

Characteristics of ohmmeters shown in block on graph.

5.6. Instruments

A number of different instruments are commer-1

cially available for measuring the resistance of con- ductive floors. Instruments fall into three general

classes: (a) Simple ohmmeters; (b) ratio ohmmeters;

and (c) Wheatstone bridges [6]. A simple d-c ohmmeter consists of a battery,

milliammeter, and a resistor, R, all in series with the

unknown resistor, X, to be measured. The currenti

through the milliammeter is adjusted (by shunting

the milliammeter or adjusting R) 6 to give full scale

deflection of the instrument, which is marked zero on the scale. The current through the instrument

is then a function of X and the scale is marked

s The internal resistance can usually be computed as the quotient of the open

circuit voltage divided by the short circuit current of the instrument.

8 It is much more desirable to shunt the milliammeter, since the calibration depends on R.

8
I

1 accordingly. Such ohmmeters with 1.5- or 3-v

batteries are commonly used in radio or television

service instruments. As indicated in section 5.1,

these voltages are far too low for meaningful tests

of conductive flooring. However, at least one

: manufacturer has incorporated a vibrator-transj former power supply operated from a dry cell to

produce a 500-v conductive-flooring tester. The moving element in a ratio-type ohmmeter

contains a permanent magnet and two rotatable

coils fixed at right angles to each other, one of which

functions as in the simple ohmmeter while the other

carries a current proportional to the applied voltage.

With no springs, the coils then take up a position

which is a function of the ratio of the voltage to the

current, and the scale is laid off in ohms or megohms.

The reading is then desirably independent of the

voltage, and a zero setting is not necessary. Most

ratio ohmmeters also contain handcranked generators

to produce the desired voltage.

Wheatstone bridges with electronic detectors have

been used occasionally, but are best suited for

laboratory work, since a balance must be made

before each reading. They are generally capable

of greater accuracy and flexibility than ohmmeters.

Table 7 shows the general characteristics of commercially available instruments which are suitable

for testing conductive flooring. In accordance with

NFPA No. 56, such instruments must have an open

circuit voltage of 500 v, a short circuit current of 2.5

to 10 ma, and should have a range of at least 0.01 to

2 meg. Unfortunately, many models of 500-v

insulation testers have a higher internal resistance

and resistance range than this. Only moderate

accuracy is required in field tests of conductive

flooring because of the large variations inherent in

most materials. An accuracy of 20 percent of the

measured resistance in the range 0.025 to 1 megshould be adequate. It can be shown that, because

the scale of an ohmmeter is necessarily nonlinear,

the markings on the scale must then be accurate to

2 percent of the full-scale length, and the resistors and

Table 7. Characteristics of typical instruments for measuring

resistance of conductive floors

Range » ApOpen- Short- Inter- proxi- Type Energy circuit circuit nal re- mate

source voltage current sistance mid- Low High scale reading

ma meg meg meg meg

Ratio ohm- Hand- 0-500 3.5 0. 14 0.005 10 0.20 meter. cranked. Ratio ohm- Hand- 0-500 10 .05 .010 50 .80 meter. cranked. Simple ohm- Internal , 500 3 . 17 .010 10 .18 meter. battery.

Simple ohm- Internal 500 2.5 .20 .010 10 .23 meter. battery. Ratio ohm- Hand- 500 5 .10 .010 50 .50 meter. cranked. Megohm 110 v a-c 500 10 b.001 <>100 bridge.

» Lowest and highest markings on scale exclusive of zero and "infinity"

markings.

b At listed short-eireuit current; additional ranges to 1,000,000 meg at lower

currents.

other component parts must be accurate to better

than 10 percent.

The data on the modified megohm bridge are included for reference. The instrument was found

to be accurate to 1 percent and well suited for

laboratory studies because of its wide voltage and

resistance ranges. However, it would not be as convenient as the others for routine tests of installed

floors.

6. Electrostatic Tests

The electrical tests of conductive flooring samples

outlined in sections 4 and 5 have been made under

deliberately varied conditions, such as relative

humidity and applied voltage, to determine the

effect of ambient conditions and of normal use upon

the samples, and to evaluate the factors which may

be expected to influence the measurements. To

substantiate these results, additional experiments

were carried out in which each type of floor was

actually used as an intercoupler to reunite electrostatic charges. These experiments provided a direct

test of the effectiveness of each type of floor in

eliminating hazards from static electricity.

Tests in the NBS laboratories have shown that a person rising from a plastic-covered chair while

wearing wool clothing causes a separation of charges

at as high a time rate (and therefore charging current) as any human action tested. A comparatively

low resistance is thus required to keep the voltage

between the objects concerned (chair and person

rising from it) below the minimum sparking voltage,

about 400 v in air at normal pressure. If the voltage

is less than this, true electrostatic sparks do not

occur [2]. This experiment therefore provides a good test of the actual performance of a floor, if the

peak voltage between the objects can be measured.

A few measurements of the voltages produced in

such tests have been made in the NBS laboratories

by using a high-resistance voltage divider, cathode

ray oscillograph, and camera.7 However, this proved

to be too cumbersome for the many tests required

for these floors, so that a simple peak-voltage indicator was constructed. It consisted of four small

sensitive neon lamps (NE2 or equivalent) connected

in series with a 40 pf capacitor across each. The

lamps and capacitors were mounted in a small

blackened metal tube having a lens at one end.

The capacitors assure that equal transient (or

alternating) voltages appear across each lamp until

one lamp fires (glows), after which all glow. Tests

showed that the firing voltage was about 300 v, somewhat less than the minimum sparking voltage.

The "dark" resistance at lower voltages ranged from

5,000 to more than 20,000 meg. Additional tests

were made later of some samples, with two neon

lamps added to the indicator, so that the firing

voltage was about 450 v, approximately that of the

standard instrument for measuring floors (500 v)

.

For these tests a person wearing conductive shoes

held the peak-voltage indicator. The other terminal

7 Since this is a transient voltage an ordinary electrostatic voltmeter can not be used.

9
of the indicator was connected to the frame of a metal chair (having flat metal gliders % in. diam)

which rested on the flooring sample.8 The person

sat in the chair, and then, while observing the indicator, rose from the chair (with a forward sliding

motion) with his feet on the same sample and noted

whether or not the lamps glowed. At least five such

trials were made with each flooring sample. The

resistance of the sample between two standard

electrodes and between the metal frame of the chair

and the subject was measured immediately after

each test at three or five different locations on the

sample.

Additional tests were made on an insulating floor

with a resistor connected between the chair and the

subject. The tests were repeated with resistors of

successively lower value until the lamps glowed

while the person rose from the chair. These tests thus determine the actual safe upper limit resistance

for the motion and materials involved.

All of the tests with the flooring samples and the

resistors were made at an ambient relative humidity

of 20 percent, with a plastic covered chair and a

subject who wore a wool suit, conditions which are very favorable to the generation of static electricity.

The results of these tests are given in table 8. The last three rows of the table show that the actual

safe limiting resistance to keep the voltage between

the chair and subject below the minimum sparking

voltage (400 v) for the motion and materials used

was about 30 megs. A resistance less than 20 to 30

megs was required to keep the voltage below 300 v, the firing voltage of the 4-lamp indicator.

The range of measured resistances between the

chair and subject are given in the fourth column of

the table. As indicated in section 5.4 this range is much greater with hard surfaced electrodes such as the leg tips on this chair than with resilient electrodes. (This is one reason why the resilient electrodes are preferred for routine tests.) The results

show that except for sample No. 5 the peak voltage

was less than 300 v when the measured resistance

between the chair and subject was less than about

30 meg, and vice versa.9 Thus there is excellent

correlation between the resistance of a conductive

floor as measured with a standard 500-v d-c instrument, and the resistance to the flow of electrostatic

charges through the same electrodes (contacting

objects).

The upper limit of resistance specified for a con- ductive floor with the standard electrode is 1 meg.

The results show that, for all samples except Nos. 5

and 11, the peak voltage is less than 300 v, if the

resistance between the two standard electrodes is less than 1 meg, and vice versa. Thus the specified

1-meg limit with two standard electrodes is a reason-

! Because of the small size of the sample only the front legs of the chair rested on the sample. Insulators were placed under the rear legs. For some of the tests with the 4S0-V indicator, two flooring samples of the same type were connected

together so that all four legs of the chair rested on the conductive floor.

• Additional tests showed that the manufacturer of sample No. 5 had applied a sealer which sometimes effectively insulated the hard-surface gliders of the

chair, but not the standard electrodes. With hard electrodes the breakdown

voltage of this insulating film ranged from 100 to more than 400 v. This sealer is no longer applied on conductive floors by the manufacturer of this sample.

Table 8. Results of electrostatic tests

Ambient temperature 25° C; relative humidity 20%; plastic chair covering; wool i garment (see text)

Sample

No. Type

Resistance (megs) between: Peak voltage

greater than: <

Two

standard

electrodes (average)

Chair and subject

(min. & max.)

300 v 450 v

2

4

S6

7

8

9

10

1112

13

15

Ceramic. _ 0.2

.9.05

. 03

6

230

1.5 0.3

.6

.1.2

0.3 to 0.5 No

Concrete. Latex.. Linoleum... ..

3 to 30

0 08 to 0 09 YesYes. No

No.

Yes.

Oxychloride 140 to 260 Yes

Oxychloride.

Oxychloride.. _

15 to 45 200 to 400 Yes... Yes

No.

Oxychloride _. Oxychloride. . Oxychloride .. _.

7 to 65

1 to 50

2 to 5

Yes...

Yes... No.Yes. No. Rubber 0.08 to 0.18 No

Vinyl .3 to 0.4 No

Resistor 20 meg No

Resistor 30 meg Yes...

Yes... No.

Resistor 50 meg Yes.

« "Yes" signifies that the designated peak voltage was exceeded at least once

in five or more trials.

ably valid criterion of the performance of these

floors.

These tests therefore show that the actual resist- i ance between the objects on which the charges are I generated (or transferred by induction) is the

important criterion of the effectiveness of a conductive floor as an electrostatic intercoupler, and that I this can be measured reasonably well with the

specified 500-v instrument. In case of doubt, as for

example floors which are slightly above the 1-meg l limit with the standard electrodes, measurements of 1 the resistance between objects in the room can ! provide additional evidence of the safety of the j floor (and the contacting objects) with respect to | the hazards from static electricity.

These tests indicate that for resilient floors there

is an appreciable "factor of safety" (about 10) for

the present 1-meg specification, even at this verylow relative humidity, because the resistance of such i floors is relatively independent of the hardness of | the contacting objects. There would appear to be j no such factor for hard-surfaced floors. However,

additional tests indicate that there is a very large

'

factor of safety (10 or more) if, as specified in NFPA

No. 56, materials such as wool and plastics are | prohibited. Such materials are excellent electro- i static generators because of their very high electrical resistivities. In addition, because the resist- \ ance of many insulating materials depends upon the j ambient relative humidity, there is another very : large factor of safety if, as recently specified in

NFPA No. 56, a relative humidity of 50 to 60 percent i is maintained. |

7. Nonelectrical Properties

7.1. Indentation

Indentation studies included the determination of

initial indentation under load and residual indenta10
tion after removal of the load. The initial indentation is sometimes referred to as the "comfort value",

in that it expresses the ability of a floor to depress

readily under foot. The value of the residual indentation, on the other hand, is an indication of the

resistance to permanent deformation due to a concentrated load, such as a table leg. The ideal floor

might thus be considered as one which has a high

initial indentation or "give" and a low residual indentation or high recovery. In table 9, linoleum,

rubber, and vinyl show a much higher initial indentation than the ceramic, concrete, and oxychloride

materials. However, other important factors are involved in determining comfort value which are of an

indirect nature, but which can be of major importance, e.g., the type of footwear involved. Comparison of these results with those reported in BMS-73

[7] show that the indentation characteristics of the

conductive materials are comparable to those of the

corresponding nonconductive materials.

Table 9. Indentation characteristics

Sample No. Type Thickness

Initial indentation Residual indentation

A B c

1 hr after removal

ol load

48 hr

after removal

of load

A B c c

12

34

5

6

7

8

9

10

1112

1314

15

Ceramic

in.

0. 25

.25 .035

.50

.443

.134

.50 .50

.56

.50

.50

.50

. 126 .121 .193

in.

0. 000

.000

.011 .000

.009

.024

.000

.000 .000

.000

.000

.002

.029

.043 .047

in. 0.000 .000

.007

.000 .007

.005

.000

.000 .000

.000

.000

.000

.008

.021

.011

in. 0.000

.000

.009

.000 .014

.007

.000 .000

.000 .000

.000

.000 .006 .024

.017

in. in. in. in.

do

Coating

Concrete

0. 003 0. 000 0. 000

Latex. .002

.003

.000

.000

.000

Linoleum .002 Oxychloride do

do

do

do

do Rubber

Vinyl —.do

0.001

.000

.002 .006 .002.000 .007 .002.001 .007

.009

.000 .000 .006

A=80-lb load applied 10 min through a 0.178-in.-diam indenter foot (3,200

lb/in.2). B=100-lb load applied 10 min through a 1.125-in.-diam indenter foot (100

lb/in.2). C = 100-lb load applied 7 days through a 1.125-in.-diam indenter foot (100

lb/in.2).

In the case of sample No. 3 (coating), the results

reflected predominantly the indentation characteristics of the plywood backing because of the thinness

of the coating material. The result would undoubtedly have been different if the same material had

been applied over concrete.

The indentation tester and procedure used in making the indentation and recovery determinations are described in detail in Report BMS-73 [7] and in

Federal Specification LLL-L—367 for Linoleum.

7.2. Scratch Resistance

Precise measurements of scratch resistance, as described in the paragraph below, were made on each

sample. This value relates to the ease with which

the floor surface can be scuffed and marred by abrasive material carried on shoes. Resistance of con- ductive flooring materials to surface scratching is of

prime importance in operating rooms for the sake of

cleanliness as well as the possible effect on electrical

contact resistance. In order to establish a basis by

which these values can be related to actual service,

each sample was placed on the pedestrian traffic test

ramp described later.

The Taber Scratch Tester was adapted to measure

the scratch width of a diamond point at loads of

250, 500, 750, and 1,000 g. A level plate capable of

travel at a uniform rate of 1 ft/min was used in

moving the flooring sample under the diamondpoint scratch tool. A scratch 2 in. long was made

and the width of the scratch was measured at three

locations with a 20 X Brinell microscope containing a scale graduated in 0.1 mm. The average of the

three measurements was converted to the nearest

0.001 in. and recorded as scratch width in mils.

The graph, figure 7, illustrates the results.

A pedestrian traffic test ramp was constructed

(see fig. 8). Each flooring sample was securely

placed on the test ramp and a photoelectric counter

was installed to count passages over the ramp. The

length of the entrance at each end of the test ramp

was altered every 3 months, causing a change in the

foot traffic pattern and therefore insuring that each

sample received the same amount of wear. The

ramp was located on the third floor of the Industrial

Building, NBS, away from street grit, gravel, and

water, but exposed to light industrial-type dirt and

dust. No attempt was made to maintain the samples

other than an occasional dry sweeping. These conditions were considered more severe than in hospital

operating rooms, resulting in an accelerated test. Photographs taken after 11 months of wear (100,000

passages) were compared with photographs of

unexposed flooring to determine the extent of

scratching, smudging, and other damage (see fig. 9).

A comparison chart of visual appearance of the

scratching on the exposed samples is listed below

the graph in figure 7.

7.3. Slippeiiness

Tests of relative slipperiness of the conductive

flooring samples were made with leather and rubber

heels under both wet and dry conditions. The

method of testing and a description of the instrument

used have been previously reported [8]. Slipperiness

is not a constant of the walkway surface or of the

contact surface of the footwear alone, but is a function of both surfaces and is materially affected by

their conditions. Therefore, an unqualified evaluation of a particular floor or floor finish may be very

misleading.

The results in table 10 show the antislip coefficients; the higher the value, the less slippery the

surface. Comparison of these results show that

the antislip characteristics of the conductive materials are comparable to those of the corresponding

nonconductive materials.

11
2 —

i a 2

0 250 500 750 1000 LOAD ON DIAMOND POINT, GRAMS

MATERIAL CERAMIC COATING CONCRETE , IMnl ...„ TERRAZZO LINOLEUM

NBS SAMPLE NO. 1 2 3 4 6

CONDITION AFTER

3 MONTHS SERVICE (33,000 PASSAGES) GOOD

1 GOOD

1 FAIR

1 1 GOOD POOR

CONDITION AFTER

II MONTHS SERVICE

(100,000 PASSAGES) GOOD GOOD FAIR GOOD POOR

MATERIAL OXYCHLORIDE RUBBER VINYL

NBS SAMPLE NO. 7 9 10 II 12 13 14 15

CONDITION AFTER

3 MONTHS SERVICE

(33,000 PASSAGES)

1 GOOD1 FAIR

1 1 GOOD GOOD

1 1 1 POOR POOR FAIR

1 FAIR

CONDITION AFTER

II MONTHS SERVICE

(100,000 PASSAGES)

FAIR FAIR GOOD GOOD POOR POOR FAIR FAIR

Figtjbe 7. Resistance to scratching.

7.4. Scrubbing

The durability of permanent type conductive floorings is quite difficult to assess and no suitable

short-term evaluation tests were available. The drytraffic tests discussed in section 7.2 and the stainresistance tests in section 7.6 were made in an attempt to judge the relative serviceability of these

materials for operating room floors. In addition,

because of the frequent washing and cleaning of

operating room floors, an apparatus was designed

to produce a continuously agitated washing effect.

The samples were placed on a plate which rotated

Figure 8. View of pedestrian traffic test ramp.

Each sample was 18 in. by 18 in.

at 4 rph and the surface of each sample was kept

continuously wet. A brush was used as the scrubber i and was rotated at 180 rpm under a total load of I 500 g. Each sample was exposed to this continuous

'

wet-brushing action for 52 hr. The samples were I examined visually by comparing the exposed with

j the nonexposed areas and by electrical conductivity

measurements.

Except for one of the oxychlorides the samples were not significantly affected by this severe scrub- 1 bing test. Sample No. 8 showed a large amount of i wear and erosion, so that the end effect was a removal i of the conductive matrix below the level of the nonconductive chips. The electrical resistance of this

sample increased to two megohms. In addition,

samples No. 9, 10, and 11 showed a slight amount

of erosion and pitting, but their electrical conductivity was not affected.

Table 10. Relative slipperiness

Sample

No. Type

Antislip coefficient

Leather heel Rubber heel

Dry Wet Dry Wet

1 Ceramic 0. 37 0.26 0. 68 0.44 K2J Ceramic . _ . .34 .26 .72 .31

3 Coating.. _ . .33 .26 .74 .46

4 Concrete terrazzo _ .43 .26 .56 .30

5 Latex terrazzo .32 .10 .73 .13

6 Linoleum .37 .18 .68 .37

7 Oxychloride..- .39 .38 .68 .48

8 Oxychloride - .36 . 15 .62 .21

9 Oxychloride .21 .18 .58 .24

10 Oxychloride .37 .14 .58 .22

11 Oxychloride . . .. .37 .12 .55 . 16 12 Oxychloride .34 .13 .54 .39

13 Rubber . . .39 .12 .67 .13

14 Vinyl .26 .16 .46 . 10

15 Vinyl .28 .13 .63 .23

12
til h . / i,X ^"•>"

' v

Figure 9. Photographs of four samples from the pedestrian traffic test ramp which were appreciably affected by the test. A, Sample No. 8 (oxychloride), showing hairline cracks; B, Sample No. 2 (ceramic), showing pitting in mortar joints; C, Sample No. 13 (rubber), showing extreme

scuffing and scratching. No wax was used; D, Sample No. 15 (vinyl), showing extreme soiling of light colored tiles. ribbon which connects the tiles electrically. One tile has been removed to show the copper

7.5. Water Absorption

It has been considered desirable that a flooring

material designated for an area subject to frequent

washing should have a very low rate of water absorption in order to reduce staining, leaching, and

erosion by water and detergents. To establish the

rates of water absorption the following test was

performed on each sample:

One-tenth milliliter of water was placed on the

sample with a pipet and covered with a watch glass.

The number of minutes required for complete absorption of the 0.1 ml of water was taken as the time of

absorption. Complete absorption was assumed to

have taken place when light no longer was reflected

from the wetted surface of the sample. All the

flooring materials except the five listed below re- quired more than 5 hr for the water to disappear and

can therefore be considered fairly irnpervious to

water.

No. Type Time of absorption min

4 Concrete terrazzo . 15

8 Oxychloride 4

9 Oxychloride 6

10 Oxychloride. 6

11 Oxychloride. 22

7.6. Stain Resistance

A high resistance to staining (or ease of cleaning

when stained) is a desirable characteristic of a

flooring material intended for use in an area where

there is likely to be spillage. Table 11 lists recommended methods for removing certain stains. Care

should be used in applying the solutions. For

example, acid solutions are usually the most effective

means of removing rust stains from concrete; 10

percent solutions of hydrochloric or phosphoric acid

are commonly used. Acids should not, however,

be allowed to remain in contact with the surface

any longer than is necessary to remove the stains.

With any such treatment, some roughening of the

concrete is inevitable and may be conspicuous. As

a precaution it is advisable to carry out a trial on a small area to determine if the resulting appearance

is acceptable. Table 12 gives results of tests

carried out to determine the effect on the conductive

floor samples of various staining agents and the ease or difficulty with which the stains were removed.

8. Summary and Conclusions

1. The conductive floors tested should give satisfactory service in hospital operating suites, with the

reservations outlined below. Similar conclusions can

be drawn from the results of extensive tests by the

staff of the National Research Council of Canada of

several types of installed floors [9]

.

13
Table 11. Stain-removal methods

Stain Linoleum Ceramic Concrete terrazzo Latex terrazzo Oxychloride Vinyl Coating Rubber

Blood

Grease or oil.

Apply cool water and rub with cloth. It stain persists, dampen cloth with ammonia.

Apply detergent

and rub with

cloth.

Apply detergent and rub with cloth. If stain persists apply kerosene or varsol on spot, permit to soak, wipe dry and wash with detergent, or mix

kerosene or varsol with fuller's earth to form paste. Apply paste to spot and let stand for several hours. Repeat if necessary.

Apply detergent and rub with cloth.

Ink (washable,

writing, and

drawing)

.

Paint or enamel

Iodine

Rust

Apply detergent and rub with cloth. If stain persists, mix 1 part sodium perborate to 25 parts water. Mix with whiting (calcium carbonate^

to form paste. Apply paste to spot and leave until dry.

Remove excess with putty knife and/or No. 0 steel wool. Apply kerosene, turpentine, or varsol. "Wash with

detergent. Remove with putty knife and No. 0 steel wool.

Apply alcohol and rub with cloth

J v, Apply alcohol and cover with fuller's earth.

Rub with No. 0

steel wool and

wash with de- tergent.

Apply water and rub with cloth. If stain persists apply solution of 1 part oxalic acid to 9 parts water. Let remain until dry. Wash thoroughly with

water.

Apply water and

rub with cloth.

If stain persists, dissolve 1 part sodium citrate in 6 parts water. Make a paste with whiting (cal- cium carbonate) and apply to

stain, or wash

with sodium ci- trate solution and

add pad of cotton soaked in sodium

bisulfite. Wash

thoroughly with

water.

Same as for linoleum.

Same as for linoleum.

-

's

Sole and heePmarking. Apply detergent and rub with cloth.

Table 12. Results on removal of stains

Sample

No. Type Blood Grease

or oil Ink

Paint

enamel

Iodine Rust

Sole and

heel marks

1 Ceramic a B B B B B B A

2 Ceramic a B B B B B B A

3 Coating B B B B B B B

4 Concrete ter- razzo _ B C C B B C B

5 Latex terrazzo with sealer. __ B B B B B B B

Latex terrazzo without sealer B B C B F C B

6 Linoleum _ B B B B B B A

7&8 Oxychloride D B D B B F B

9 Oxychloride ... F F F B B B B

10 Oxychloride ... B F D B A F B

11 Oxychloride B B D B F B A

12 Oxychloride ... D F D B B B B

13 Rubber B B B B B C B

14 Vinyl B B B B C B C

15 Vinyl _. B B C B C B C

Note: A, Staining material would not stain or mark floor; B, stain completely

removed with little effort; C, stain completely removed, but with difficulty: D, stain completely removed, but floor surface bleached or etched; F, stain not removed.

* Results on cement joints same as No. 4, concrete terrazzo.

2. In general, any particular type of conductive

flooring may be expected to render service comparable to nonconductive flooring of the same type.

Consequently, an architect may base his choice of a conductive flooring material on his knowledge of

the behavior of similar nonconductive materials,

with the following limitations

:

(a) The durability and appearance of at least

two of the available materials (linoleum and rubber)

may depend on periodic waxing. Conductive waxes

containing carbon black which do not deposit an

insulating film are available and should be estheti

cally as well as electrically satisfactory on these uni

formly black floors. Sealers should not be used on

conductive floors until proven satisfactory by ex- tensive electrical tests. (b) Since the most commonly used conductive

medium (acetylene black) is black, the colors available in most types of conductive floors are limited

However, terrazzo and other pattern effects can be' used. Detailed descriptions of some of the available colors and patterns are given in section 3. (c) The electrical resistance of the oxychloride

floors depended on their moisture content, which in|

turn was governed by the humidity of the air and'

by water added during washing. The results or

the tests given in this report indicate that if this,

material is used the relative humidity of the ah in

the room in which it is installed should be controlled

and the cleaning schedule for the floor should be

carefully established and maintained. The labora-|

tory tests indicate that otherwise the electrical re- sistance of the floor may fall outside the accepted'

limits.

3. The results of this investigation indicate that!

the presently accepted method of measuring the

resistance of installed floors, described in NFPA

No. 56, reasonably simulates the conditions unde^

which a floor is expected to function as an electrostatic intercoupler.

14
f 4. The results also show that current specifications

jhnd methods of measuring the physical properties

jpf nonconductive floorings are satisfactory for conductive floorings.

The work described in this report was done under

! a, project sponsored jointly by the Office of the Chief

of Engineers, Department of the Army; Bureau of

'i Yards and Docks, Department of the Navy; and

'Engineering Division, Director of Civil Engineering,

^Department of the Air Force.

The authors appreciate the cooperation of the

[staff of the Division of Building Research of the

National Research Council of Canada, particularly

Mr. P. J. Sereda, in making available their experience

with conductive flooring. The cooperation of the

numerous manufacturers who generously provided

samples is also acknowledged.

9. References

[1] P. G. Guest, V. W. Sikora, and B. Lewis, Static elec- tricity ir hospital operating suites: Direct and related

hazards and pertinent remedies, Bureau of Mines

Report of Investigation 4833, 64 pp. (1952).

[2] F. B. Silsbee, Static electricity, NBS Circ. 438 (June

1942).

[3] F. L. Hermach, Hazards from static electricity, The

Military Engineer 44, 287 (1952).

[4] Recommended safe practice for hospital operating rooms,

N.F.P.A. No. 56, National Fire Protection Association

(reproduced in part in appendix A of this report).

[5] Robin Beach, Electrostatic safety for hospital operating

rooms, Elec. Eng. 72, 329 (April 1953).

[6] AIEE master test code for resistance measurements,

Publication No. 550, Am. Inst. Elec. Engrs., New

York, N.Y.

[7] Percy A. Sigler and Myrtle B. Woodward, Indentation of

floor coverings, NBS BMS Report 73, 7 pp. (1941).

[8] Percy A. Sigler, Martin N. Geib, and Thomas H. Boone,

Measurement of the slipperiness of walkway surfaces,

J. Research NBS 40, 339 (May 1948) RP1879.

[9] P. J. Sereda, Properties of commercial conductive flooring

for hospital operating rooms, Division of Building

Research, Nat. Research Council Can., NRC 4031.

10. Appendix A. Excerpts 10 from the 1958

Edition of NFPA Publication No. 56,

Recommended Safe Practice for

Hospital Operating Rooms

6-2. Conductive Flooring.

(a) Resistance.

1. To prevent the accumulation of dangerous

electrostatic charges, the surface of the floor . . . shall provide a path of moderate electrical conductivity between all persons and equipment making

contact with the floor. No point on a non-conductive element in the surface of the floor shall be

more than % inch from a conductive element of the

surface.

NOTE: It is not necessary to provide a special

floor-to-ground connection unless required by

the authority having jurisdiction.

2. The resistance of the conductive floor shall be

io Reproduced by permission of the National Fire Protection Association, 60 Batterymarch Street, Boston, Massachusetts.

less than 1,000,000 ohms as measured between two

electrodes placed three feet apart at any points on

the floor.

3. For additional protection against electric shock,

the resistance of the floor shall be more than 25,000

ohms, as measured between a ground connection

and an electrode placed at any point on the floor,

and also as measured between two electrodes placed

3 feet apart at any points on the floor.

(b) Method of Test.

1. The floor shall be clean and dry and the room

shall be free of explosive gas mixtures. Each electrode shall weigh 5 pounds and shall have a dry, flat,

circular contact area 2)i inches in diameter which

shall comprise a surface of aluminum or tin foil

0.0005 to 0.001 inch thick backed by a layer of

rubber }{ inch thick and measuring 50 plus or minus

10 hardness as determined with a Shore Type A

durometer. (American Society for Testing Materials Tentative Method of Test for Indentation of

Rubber by Means of a Durometer, ASTM Designation D676^9T, obtainable from ASTM, 1916

Race St., Philadelphia 3, Pa.)

2. A suitably calibrated ohmmeter with a nominal

open-circuit output voltage of 500 volts D.C. and

a short-circuit current of 2.5 to 10 milliamperes

shall be used. Measurements* shall be made at

five or more locations in each room and the results

averaged. For compliance with Section 6-2 (a) 2,

the average shall be within the limits specified and

no value shall be greater than 5 megohms. For

compliance with Section 6-2 (a) 3, no location shall

have a resistance of less than 10,000 ohms and the

average for not less than five locations shall be

greater than 25,000 ohms. Where resistance to

ground is measured, two measurements shall be

made at each location, with the test leads interchanged at the instrument between measurements,

with the average to be taken as the resistance to

ground at that location. All readings may be taken

with the electrode or electrodes more than 3 ft. from any ground connection or grounded object

resting on the floor.

*NOTE: If the resistance changes appreciably

with time during a measurement, the value

observed after the voltage has been applied for

about 5 seconds shall be considered to be the

measured value.

13-10. Testing and Maintenance

(a) Conductive Floors

1. The resistance of conductive floors shall be

initially tested prior to use, by the method described

in Section 6. Thereafter measurements shall be

taken at intervals of not more than one month. A

permanent record of the readings shall be kept.

2. To be effective the surface of conductive floors

shall not be insulated by a film of oil or wax. Any

waxes, polishes, or dressings used for maintenance

of conductive floors shall not adversely affect the

resistance of the floor.

15
3. Cleaning procedures for conductive floors shall

be established, then carefully followed to assure that

conductivity characteristics of the floors are not

adversely affected by such treatment.

A-6-2. Conductive Flooring

In hazardous locations, a conductive floor serves

as a convenient means of electrically connecting

persons and objects together to prevent the ac- cumulation of electrostatic charges. A resistance

not exceeding 5 to 10 megohms between the objects

or persons is generally sufficient to prevent dangerous

voltages. The upper limit of 1,000,000 ohms for

the resistance of the floor has been chosen as meeting

this requirement with a reasonable factor of safety

and with reasonable provision for other resistances

in the conductive path.

The conductive floor produces a definite shock

hazard in the absence of an isolated electrical system.

The isolating transformers must always be installed

simultaneously with or prior to the installation of

the conductive floor. Installation in the opposite

sequence is definitely dangerous and should not be

contemplated.

The resistance of some flooring materials increases

with age. Floors of such materials should have

an initial resistance low enough to permit increases

in resistance with age without exceeding the limits

prescribed in Section 6-2 (a)2.

A conductive floor need not be provided with a

special grounding connection to prevent the accumulation of charges due to the motion of objects or persons resting on it. To be effective it is necessary

only that it be conductive and that the persons and

objects be electrically connected to it. Considerable

conductivity to ground is generally attained in the

usual construction, often because of the proximity

of grounded conduits and water pipes. This incidental conductivity to ground and the large area,

and therefore capacitance of the floor, make any

hazard due to the entry of charged persons or objects

into the protected zone negligible, provided such

persons and objects have proper conductivity to the

floor. Grounded objects within the room, such as portable electrical equipment, will be electrically

connected to the floor by the requirements of Section

6, and will thus ground the floor, so that such

objects do not increase this electrostatic hazard.

Therefore, a special grounding connection need not

be incorporated in the floor for electrostatic protection, unless desired or otherwise required.

11. Appendix B. Four-Terminal Method of

Measuring Contact and Internal

Resistivities

If rectangular current electrodes, Ci and C2 , and

very narrow potential electrodes, P! and P2 , are placed parallel to each other on the same surface of a rectangular specimen of uniform internal or volume resistivity, r, as shown in figure 5, all equipotential lines are also parallel to the electrodes. Then,

if V, E, and the total resistance, R, are measured,

we have I=V/R=E/Q=Elb/rf where Q is the resist- ance between P1 and P2

. Thus r=ERlb/Vf (in

ohm-cm if R is in ohms and all dimensions are in j centimeters).

The contact resistivity, s, may be defined as the re- ! sistance of a unit area of the interface between the j current electrodes and samples. If the internal resistj ance of the sample under each current electrode is

| neglected,11 we may compute the internal resistance,

T, of the sample to be approximately T=rd/lb =

ERd/Vf. The contact resistance under one electrode

is S—s/wl where wl is the apparent contact area of

the electrode. Since, under these simplifying assumptions, R=TJ

r2S=TJ

r2slwl, we have in ohm-cm2

12. Appendix C. Calculation of Resistance

From Contact and Internal Resistivities

The resistance between two NFPA electrodes of

diameter d spaced D centimeters apart (D^>^>d) on

a large floor may be evaluated by adding together

the contact resistance under both electrodes and the

internal resistance of the floor between two imaginary cylinders which form the downward projections

of the electrodes through the floor (again neglecting

the resistance of the material under the electrodes).11 The internal resistance may be calculated by using

established formulas of the capacitance per unit

length between two long cylinders of the same diameter and spacing and applying a conversion formula

to find the internal resistance T. In cgs electrostatic

units this capacitance, for a length b (corresponding

to the thickness of the floor), is

where In is the natural logarithm.

The conversion formula from which the internal

resistance can be computed can be expressed as T=rl4xG. Thus

The sum of the contact resistances for both electrodes is 2S=8s/wd2 and the total resistance between

electrodes is approximately

D 2.5s,0.7r, 2D R=^+-Tlog ~d'

where log signifies the logarithm to the base 10.

The above relations presuppose a uniform material

having a negligible voltage coefficient of resistance.

Thus, they are only approximate.

" Approximate mathematical investigation shows that because of the high

ratio of contact to internal resistivity of these flooring materials, the error intro- duced by this assumption Is much smaller than the influence of other factors, such as the nonuniformity and the voltage coefficient of resistivity.

Washington, D.C.

16 U.S. GOVERNMENT PRINTING OFFICE:1960


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